1. Field of the Invention
The present invention relates to a solid-state imaging device, an imaging apparatus, and an AD conversion gain adjusting method.
2. Description of the Related Art
For example, in the field of video apparatuses, CCD (Charge Coupled Device), MOS (Metal Oxide Semiconductor), or CMOS (Complementary Metal-Oxide Semiconductor) solid-state imaging devices that detect light (an example of an electromagnetic wave) among physical quantities are used. The solid-state imaging devices read out, as an electric signal, a physical quantity distribution converted into the electric signal by unit components (pixels in the solid-state imaging devices).
There is also an amplification solid-state imaging device including pixels of an amplification solid-state imaging element (APS: Active Pixel Sensor/also referred to as gain cell) configuration having a driving transistor for amplification in a pixel-signal generating unit that generates a pixel signal corresponding to a signal charge generated by a charge generating unit. For example, most of CMOS solid-state imaging devices have such a configuration.
For example, in an X-Y address type solid state imaging device in which unit pixels are arranged in a matrix shape, in order to impart an amplification function to pixels, the pixels are configured by using active elements (MOS transistors) of a MOS structure or the like. Specifically, signal charges (photoelectrons) accumulated in photodiodes serving as photoelectric conversion elements are amplified by the active elements and read out as image information. For example, a large number of pixel transistors are arrayed in a two-dimensional matrix shape to configure a pixel unit. Accumulation of signal charges corresponding to incident light is started for each of rows or each of pixels. Signals of electric currents or voltages based on the accumulated signal charges are read out in order from the pixels according to address designation.
In the MOS (including CMOS) solid-state imaging devices, as an example of address control, a column readout system (a column parallel output system) for simultaneously accessing all pixels for one row and reading out pixel signals from a pixel unit in row units is often used. An analog pixel signal for one row read out from the pixel unit may be converted into digital data by an analog to digital converter (an AD converter/ADC). Various mechanisms for AD conversion are proposed.
As an AD conversion system, various systems are devised from viewpoints of a circuit size, processing speed (an increase in speed), resolution, and the like. For example, there is an AD conversion system of a reference signal comparison type (see JP-A-2007-60671 and JP-A-2007-60080). The reference signal comparison type is also referred to as a slope integral type, a ramp signal comparison type, and the like.
In the AD conversion system of the reference signal comparison type, a reference signal (a ramp wave), a value of which gradually changes, is used for voltage comparison for converting analog data into digital data. An analog unit signal and the reference signal are compared. Digital data of a unit signal is acquired on the basis of a count value obtained by performing count processing in a count operation effective period based on a comparison processing result. In a system obtained by combining the AD conversion system of the reference signal comparison type and the column readout system (referred to as column AD system, an analog output from pixels can be subjected to AD conversion in a low band in a column-parallel manner. This is suitable for an image sensor that realizes both a high image quality and high speed.